Chae Hyun Uk, Ahsan Ragib, Tao Jun, Cronin Stephen B, Kapadia Rehan
Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, California 90089, United States.
Nano Lett. 2020 Mar 11;20(3):1791-1799. doi: 10.1021/acs.nanolett.9b05023. Epub 2020 Mar 2.
Here, we show that the turn-on voltage for the hydrogen evolution reaction on a graphene surface can be tuned in a semiconductor-insulator-graphene (SIG) device immersed in a solution. Specifically, it is shown that the hydrogen evolution reaction (HER) onset for the graphene can shift by >0.8 V by application of a voltage across a graphene-AlO-silicon junction. We show that this shift occurs due to the creation of a hot electron population in graphene due to tunneling from the Si to graphene. Through control experiments, we show that the presence of the graphene is necessary for this behavior. By analyzing the silicon, graphene, and solution current components individually, we find an increase in the silicon current despite a fixed graphene-silicon voltage, corresponding to an increase in the HER current. This additional silicon current appears to directly drive the electrochemical reaction, without modifying the graphene current. We term this current "direct injection current" and hypothesize that this current occurs due to electrons injected from the silicon into graphene that drives the HER before any electron-electron scattering occurs in the graphene. To further determine whether hot electrons injected at different energies could explain the observed total solution current, the nonequilibrium electron dynamics was studied using a 2D ensemble Monte Carlo Boltzmann transport equation (MCBTE) solver. By rigorously considering the key scattering mechanisms, we show that the injected hot electrons can significantly increase the available electron flux at high energies. These results show that semiconductor-insulator-graphene devices are a platform which can tune the electrochemical reaction rate via multiple mechanisms.
在此，我们表明，在浸入溶液中的半导体 - 绝缘体 - 石墨烯（SIG）器件中，石墨烯表面析氢反应的开启电压可以被调节。具体而言，研究表明，通过在石墨烯 - 氧化铝 - 硅结两端施加电压，石墨烯的析氢反应（HER）起始电位可偏移超过0.8 V。我们发现这种偏移是由于从硅隧穿到石墨烯导致石墨烯中产生了热电子群体。通过对照实验，我们表明石墨烯的存在对于这种行为是必要的。通过分别分析硅、石墨烯和溶液的电流分量，我们发现在固定的石墨烯 - 硅电压下，硅电流增加，这对应于HER电流的增加。这种额外的硅电流似乎直接驱动了电化学反应，而不改变石墨烯电流。我们将这种电流称为“直接注入电流”，并推测这种电流是由于从硅注入到石墨烯中的电子在石墨烯中发生任何电子 - 电子散射之前驱动了HER。为了进一步确定以不同能量注入的热电子是否可以解释观察到的总溶液电流，我们使用二维系综蒙特卡罗玻尔兹曼输运方程（MCBTE）求解器研究了非平衡电子动力学。通过严格考虑关键散射机制，我们表明注入的热电子可以显著增加高能下的可用电子通量。这些结果表明，半导体 - 绝缘体 - 石墨烯器件是一个可以通过多种机制调节电化学反应速率的平台。
